A variety of commercial processes employ ultraviolet (UV) radiation for treating materials. For example, ultraviolet radiation is used to polymerize photopolymer coatings. Photopolymer coatings find widespread use as protective surface coatings, printing inks and in the production of printed circuits.
Ultraviolet lamps have been developed for irradiation of photocurable coatings. The ultraviolet lamp typically utilizes a bulb fill which contains mercury together with various additives to emphasize a particular region or regions of the light spectrum.
In many instances, the geometry or thickness of the coating requires a rich spectrum of radiation be applied if efficient polymerization is to be achieved. More particularly, the surface of the coating typically requires shorter wavelength photons be applied for efficient absorption within the first several molecular layers of the coating thereby insulating the film from the loss of the activated chemical species to the surrounding media. The bulk of the coating requires somewhat longer wavelength photons that have the ability to penetrate the coating more deeply than do the shorter wavelength photons. As is apparent, the spectral energy density distribution is empirically determined prior to application and the appropriate spectrum of irradiation selected. Since prior art UV lamps are unable to provide regions of the spectrum that are carefully tailored to the photochemical requirements of many photocurable coatings, a compromise spectrum is usually selected or in the alternative, two lamps are used for the cure, one emitting predominately longer wavelengths while the other optimized to emit predominately shorter wavelengths. This results in an inferior, less desirable and lower quality cure of the coating not to mention the added cost for having to use two separate lamps for the cure.
In recent years, discharge devices which emit so-called excimer radiation have become known. Excimers are unstable excited molecules that possess an unbound or weakly bound ground state. That is, the excimer molecules exist only in the excited states. The excimer molecules disintegrate within less than a microsecond, and during their decay give off their binding energy in the form of radiation in a relatively narrow band.
U.S. Pat. No. 5,504,391 to Turner et al. discloses a class of lamps that produces a single narrow band of emission frequencies. Since this prior art lamp is based upon an excimer emission systems, the spectral power density of the lamp, normalized to the excimer emission peak, has been shown to be invariant with respect to the excitation power. Consequently, the only variable that the operator has any control over is total power. Further, and for the reasons earlier stated, single narrow band emissions are suboptimally matched to many commercial applications and in particular photopolymerization applications requiring a rich spectrum for efficient polymerization.
In summary, spectral adjustment of prior art high pressure, high voltage ignited microwave bulbs containing a single rare gas-halogen excimer is limited to variation of light intensity with power input and a minor variation in the line shape of the dominant transitions by varying the bulb fill. As it is apparent, such adjustments fall short from that desired for many applications.